Active implantable medical device with dynamic optimization of stimulation pulse energy

ABSTRACT

The disclosure relates to a device including a circuit for adjusting the energy of the stimulation pulses, independently controlling the pulse width and the voltage of each stimulation pulse. An iterative search algorithm for determining the optimum energy includes changing both the pulse width and voltage at each new pulse delivered, by setting a high energy value and a low energy value, and delivering a stimulation pulse with the low energy value. A capture test is then carried out. In the presence of a capture, a current iteration is complete and a new iteration is done with the current low energy as a new high energy value. In the absence of capture, the algorithm is terminated with selection of the last energy value that produced the capture as the value of optimum energy.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims the benefit of and priority to French PatentApplication No. 1550203, filed Jan. 12, 2015, which is incorporatedherein by reference in its entirety.

BACKGROUND

The invention relates to “active implantable medical devices” as definedby the Directive 90/385/EEC of 20 Jun. 1990 of the Council of theEuropean Communities, and particularly to implantable devices thatcontinuously monitor heart rate and if necessary deliver electricalstimulation, resynchronization and/or defibrillation pulses to the heartin case of rhythm disorder detected by the device.

The invention relates especially, but is not limited to, those devicesthat are in the form of an autonomous capsule intended to be implantedin a heart chamber, including the ventricle.

These capsules are free of any mechanical connection to an implantable(such as a housing of the stimulation pulse generator) ornon-implantable (external device such as programmer or monitoring devicefor patient remote monitoring) main device, and for this reason arecalled “leadless capsules” to distinguish the capsules from electrodesor sensors disposed at the distal end of a conventional probe (lead),which is traversed throughout its length by one or more conductorsgalvanically connecting the electrode or sensor to a generator connectedto an opposite, proximal end of the lead. A detection/stimulationelectrode in contact with the wall of the ventricle enables the capsuleto detect the presence or absence of a spontaneous depolarization waveof the cardiac cavity, as well as the occurrence time of the wave(ventricular or atrial marker).

The electrode also allows the delivery of a stimulation pulse in theevent of absent or late spontaneous depolarization, so as to causecontraction of the cardiac cavity.

Note, however, that the autonomous nature of the capsule is notinherently a necessary feature of the present invention.

The management of the stimulation energy is a critical aspect of anyimplantable pacemaker, because it has a direct impact on the powerconsumption of the integrated pacemaker battery, and thus on its overalllifespan.

This topic is particularly critical in the case of a leadless capsulepacemaker wherein, unlike conventional pacemakers, the energy requiredfor the issuance of stimulation is 70% of the total energy consumed. Inaddition, it must be considered that the very small dimensions of aleadless capsule imposes minimizing the size of the battery and thus itscapacity, as the battery often occupies more than 70% of the volume in aleadless capsule.

In fact, if it was possible to reduce, for example, half the energyrequired for stimulation, the size of the battery could correlatively bereduced about 40% while keeping the same longevity, which would reducethe volume of the capsule to about 0.6 cm³ (compared to 1 cm³ in thebest case today), all performances being equal.

To minimize the energy dedicated to stimulation as much as possible,while maintaining the effectiveness of delivered electrical pulses, atechnique called “cycle to cycle capture” may be employed. Cycle tocycle capture maintains the stimulation energy at a minimum level,continuously checking, after each stimulation, if the stimulation waseffective (“capture”) or not. If no depolarization wave has been inducedby stimulation of the cardiac cavity (“non-capture”), the implantdelivers, during the same cardiac cycle, a stimulation of a relativelyhigh energy to ensure the triggering of a depolarization. Then, bysuccessive iterations, the stimulation energy is gradually reduced ineach cardiac cycle, so as to converge again to an energy close to thelimit or “triggering threshold” needed to cause depolarization of thecardiac cavity.

The invention relates more precisely to a method to determine the pacingthreshold by successive approaches, in the most efficient possiblemethod from the energy consumption point of view.

The basic technique which is commonly used today in most pacemakers, isdescribed in U.S. Pat. No. 3,777,762 A. The technique involves using amethod of progressive decreases in amplitude (voltage) of thestimulation pulses for a fixed pulse width.

Another technique is described in U.S. Pat. No. 4,979,507 A. Thistechnique relies on the fact that the delivered energy not only dependson the amplitude of the stimulation pulses, but also of the width ofthese pulses (stimulation duration). The pacing threshold varies as afunction of these two parameters according to a nonlinear law called“Lapicque law”.

The technique proposed in U.S. Pat. No. 4,979,507 A includes performingtwo amplitude scans, with two different pulse widths. This approach hasa risk of capture default, because the theoretical Lapicque law definesa boundary between capture and non-capture that, in practice, variesfrom one patient to another. It is therefore necessary to validateeither continuously or at regular intervals the method for each patient,by making a complete scan of all possible values of the parameters(amplitude and width of the stimulation pulse). However, a full scan isimpractical because it is very costly in terms of energy and requiresinterrupting therapy during scanning.

WO 94/12237 Al discloses another technique for automatically adjustingthe capture threshold wherein, again, the variation of the energy of onestimulation pulse to the next is made either by changing the duration ofthe pulse, or by changing the amplitude of the pulse. This significantlyincreases the number of iterations required for the search algorithm todetermine the actual value of the stimulation threshold.

U.S. Pat. No. 5,718,720 A, U.S. Pat. No. 5,702,427 A, U.S. Pat. No.5,549,652 A and U.S. Pat. No. 6,650,940 B1 describe other techniques fordetermining the pacing threshold, implementing various capture detectionmethods such as a direct detection of mechanical myocardial contraction,analysis of an accelerometric signal, analysis of a temperature signal,analysis of intracardiac pressure, etc.

SUMMARY

The object of the disclosure is to provide a new technology to searchfor an optimum of both parameters defining the energy delivered by thestimulation pulse, namely the stimulation voltage (the amplitude of thepulse) and the duration of the stimulation (the width of the pulse), inboth the fastest and the most energy consumption saving method.

The problem to solve is minimizing the number of stimulations to deliverto determine the pacing threshold, so as to consequently reduce thepower consumption of the implant in order to improve the overalllifespan.

The starting point of the disclosure is, in contrast to known searchtechniques which typically operate by scanning successive amplitudevalues for a given pulse width, simultaneously executing a searchalgorithm in two dimensions (width and amplitude pulse). This algorithmallows for the possibility of varying both parameters of a stimulationpulse to the next stimulation pulse according to a mechanism thatdepends on the result (presence or absence of capture) of the previousstimulation.

As will also be seen, the disclosure provides such an algorithmiteratively operating by dichotomy, on the basis of a minimization ofthe total energy of the pulse, and not only the minimization of thevoltage of the pulse.

More specifically, the invention proposes an active implantable medicaldevice including:

-   a ventricular stimulation circuit adapted to deliver low energy    pacing pulses to an implantable electrode within a heart chamber of    a patient;-   a capture test circuit adapted to detect, during a cardiac cycle,    the presence or absence of a contraction subsequent to the    application of a stimulation pulse; and-   an adjusting circuit capable of independently controlling the    stimulation voltage and the stimulation pulse width of the energy    pulses delivered by the stimulation circuit.

In one embodiment, the adjustment circuit is configured to implement aniterative algorithm to re-search for optimum energy and is capable ofmodifying both the pulse width t and the voltage V of each new deliveredpulse. The adjustment circuit is configured to, at each currentiteration, perform the following actions:

-   set a value {t,V} of high energy;-   set a value {t′,V′} of low energy, with t′<t and V′<V;-   deliver a pacing pulse with the low energy value, then perform a    capture test; and    -   in the presence of a capture, end the current iteration and        transition to a new iteration, with the current low energy as        the new high energy value,    -   in the absence of capture, i) apply a consecutive r stimulation        pulse of pulse width t and of voltage V defined for said high        energy value, and ii) the algorithm and select the last energy        value that produced the capture as the optimum energy value.

In a preferred embodiment, the adjustment circuit is further configuredto perform the following actions:

-   set a first intermediate energy value {t′,V};-   set a second intermediate energy value {t, V′};-   set a third intermediate energy value {t″,V″}, with t′<t″<t and    V″<V″<V-   rank the first, second and third intermediate energy values by    decreasing energy value; and-   in the absence of capture after delivery of the pulse with low    energy value and capture test, continue the current iteration with    delivery of pacing pulses in succession with the first, second and    third intermediate energy values sorted by decreasing value of    energy to detect a capture; and    -   in the presence of a capture, end of the current iteration and        transition to a new iteration with the current intermediate        energy that produced the capture as a new high energy value,    -   in the absence of capture, complete the algorithm and selection        of the last value of energy produced with the capture among the        first, second and third intermediate energy values as the        optimum energy value.

The third intermediate energy value may be a value {t″,V″} such thatt″=(t+t′)/2 and V″=(V+V′)/2.

According to various advantageous subsidiary embodiments:

-   the energy values of the pulses delivered by the stimulation circuit    are, at most, equal to a maximum energy limit value, and the high    energy value {t,V} in the first iteration of the algorithm is the    maximum energy limit value;-   the energy values of the pulses delivered by the stimulation circuit    are at least equal to a minimum energy limit value {tL,VL} (L),    wherein said low energy value is a value {t′,V′} such that    t′=(t+tL)/2 and V′=(V+VL)/2;-   the energy values of the pulses delivered by the stimulation circuit    are between a maximum energy limit value and a minimum energy limit    value calculated before each first iteration of the algorithm;-   in the latter case, the pulse width and the voltage of the maximum    energy value and of the minimum energy value are calculated by the    application of multiplication factors, respectively the upper and    lower unit of the current pulse width and of the current voltage of    the stimulation circuit before the first iteration of the algorithm.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, characteristics and advantages of the presentinvention will become apparent to a person of ordinary skill in the artfrom the following detailed description of preferred embodiments of thepresent invention, made with reference to the drawings annexed, in whichlike reference characters refer to like elements and in which:

FIG. 1 is an overall perspective view of a leadless capsule.

FIG. 2 is a longitudinal cross sectional view of the leadless capsule ofFIG. 1 showing the main internal components.

FIG. 3 is a series of timing diagrams illustrating an electrogram EGMsignal, the detection windows for the capture test and the endocardialacceleration EA signal.

FIG. 4 is a three-dimensional representation of the energy expended bythe application of a stimulation pulse, depending on the amplitude andwidth of the stimulation pulse.

FIG. 5 is a two dimensional representation, as a function of theamplitude and the width of successive stimulation pulses, of thedichotomy search technique according to the disclosure, with, for eachiteration, concurrently changing the amplitude and the width of thedelivered pulse.

FIG. 6 is a representation of the algorithm of FIG. 5 applied to a firstillustrative implementation.

FIG. 7 is a voltage/pulse width diagram corresponding to the example ofFIG. 6, to which isoenergetic curves have been added.

FIG. 8 is similar to FIG. 6, applied to a second illustrativeimplementation of the disclosure.

DETAILED DESCRIPTION

An exemplary embodiment of the device of the disclosure will now bedescribed.

Regarding its software aspects, the disclosure may be implemented byappropriate programming of the controlling software of a known cardiacpacemaker, for example an endocardial leadless capsule.

These devices include a programmable microprocessor provided withcircuits for shaping and delivering stimulation pulses to implantedelectrodes. It is possible to transmit software to the device bytelemetry that will be stored in memory and executed to implement thefunctions of the disclosure which will be described below. Theadaptation of these devices to implement the functions of the disclosureis within the reach of a skilled-in-the-art person and will not bedescribed in detail. In particular, software stored in memory andexecuted can be adapted and used to implement the functions of thedisclosure which will be described below.

The method of the disclosure is implemented primarily by software,through appropriate algorithms performed by a microcontroller or adigital signal processor. For the sake of clarity, the variousprocessing applied will be decomposed and schematized by a number ofseparate functional blocks in the form of interconnected circuits, butthis representation, however, is only illustrative, these circuitsincluding common elements in practice correspond to a plurality offunctions generally performed by the same software.

FIGS. 1 and 2 respectively show, in perspective and in longitudinalcross section, an example of a leadless capsule.

In these figures, the reference 10 generally designates the capsule,formed as a cylindrical tubular body 12 of axis 4 enclosing the variouselectronic circuits and power supply of the capsule. Typical dimensionsof such a capsule are a diameter of about 6 mm and a length of about 25mm.

At its distal end 14, the capsule includes a helical anchoring screw 16for fixing the capsule into tissue, for example against a wall of aheart chamber. The helical anchoring screw 16 can optionally be anactive, electrically conductive screw for collecting the potential ofcardiac depolarization and/or for the application of stimulation pulses.The proximal region 18 of the capsule 10 has a rounded, atraumatic end20 and is provided with grips 22, 24 suitable for implantation orremoval of the capsule.

As shown in FIG. 2, the capsule 10 incorporates a battery 26, typicallywith a volumetric energy density of the order of 0.8 to 2 kg/cm³, anelectronic module 28, a front electrode 30, and optionally a sideelectrode 32. Feedthroughs such as 34 are used to connect the electrodesto the electronic module 28.

The electronic module 28 includes all of the electronics for controllingthe various functions of the implant, storing the collected signals,etc. It includes a microcontroller and an oscillator generating theclock signals necessary to the operation of the microcontroller andcommunication. It also contains an analog/digital converter and adigital storage memory. It may also contain a transmitter/receiver forexchanging information with other implantable devices by HBC (Human BodyCommunication, intracorporeal communication) communication.

The capsule 10 also includes a endocardial acceleration (EA) sensor 36capable of delivering a signal representative of the mechanical activityof the myocardium, for example a microaccelerometer shaped sensorinterfaced with the electronic module 28.

The sensor of EA signal 36 can be a 1D, 2D or 3D accelerometric sensor.Preferably, the sensor is a piezoelectric or a capacitive sensor, butother types of sensors (optical, resistive, inductive, etc.) capable ofgenerating a signal correlated to the displacement, velocity oracceleration of the heart walls may be used.

FIG. 3 shows a series of timing diagrams illustrating an electrogram(EGM) signal, detection windows W_(DET) for the capture test, and theendocardial acceleration (EA) signal.

After each stimulation (marker V of stimulated depolarization on theEGM), the measurement of the EA signal delivered by the accelerometer isactivated for a W_(DET) window which is open either immediately afterthe issuance of the stimulation pulse, or with a delay δ on the order of5 to 100 ms. The length F of the window W_(DET) is between 75 and 350ms. Controlling the start time of the capture window W_(DET) and itsduration is achieved via a sequencing circuit of the microcontroller andthe embedded software which controls the electronic circuits of theimplant.

EP 2412401 A1 (Sorin CRM) discloses a capture test technique byanalyzing a signal EA, including successive components (EA components)of the signal which correspond to the major heart sounds that can berecognized in each cardiac cycle (S1 and S2 sounds of aphonocardiogram). The amplitude variations of the first component (EA1component) are closely related to changes in pressure in the ventricle,while the second component (EA2 component) occurs during theisovolumetric ventricular relaxation phase. The analysis can also takeinto account the secondary component (called EA4 or EA0) produced by thecontraction of the atrium.

These components are analyzed to extract various relevant parameterssuch as the peak-to-peak of the PEA1 and PEA2 peaks of the EA1 and EA2components, the temporal interval between these PEA1 and PEA2 peaks, thehalf-height width of the EA1 and/or EA2 components, the instants ofbeginning and ending of these components, etc. It may also berepresentative of morphological parameters of the waveform of the EAsignal or of its envelope.

This capture technique by analyzing an EA signal is not, however,limitative of the disclosure and one can for example proceed asdescribed for example in EP 0552357 A1 (ELA Medical) by analysis of EGMsignals of depolarization of the myocardium to recognize the presence orabsence of an evoked wave consecutive to the application of thestimulation pulse.

The basic concept of the disclosure, unlike known techniques which oftenoperate a scanning of the amplitude of the stimulation pulse at constantpulse width, is to operate a search algorithm simultaneously in twodimensions (amplitude and pulse width).

The energy expended by the delivery of a stimulation pulse amplitude ofvoltage V and of width t is given by:

${E\left( {V,t} \right)} = \frac{V^{2}t}{R}$

R being the impedance of the heart tissue between the two stimulationelectrodes.

FIG. 4 shows the variation of the energy E expended by a pacing pulse asa function of the two parameters V and t. This representation includestwo areas, with a capture zone ZC, wherein the energy delivered issufficient to cause myocardial contraction, and a non-capture area ZNC,wherein this stimulation energy was not sufficient to cause myocardialcontraction. These two zones are separated by a border CL, correspondingto the theoretical Lapicque's curve, which is a nonlinear theoreticalboundary that may vary from one patient to another. In the capture zoneZC, the stimulation energy increases with the voltage and the pulsewidth, according to a nonlinear relation.

The energy E(V, t) is the power actually dissipated in the impedance R,that is to say, in the heart tissue. The energy actually consumed by theelectric power source, E_(p) (V, t), of the implant (battery orrechargeable battery) is equal to:

${E_{p}\left( {V,t} \right)} = \frac{V^{2}t}{{\eta (V)}R}$

wherein r_(i)(V) is the yield of the circuit for generating thestimulation voltage V.

The search technique of optimum energy by dichotomy according to thedisclosure will now be explained with reference to FIG. 5.

The purpose is to achieve, in a minimum number of steps, the stimulationconditions (pulse amplitude and width) that minimize the energynecessary for the issue of pulses providing an effective capture.

It is assumed that the stimulation circuit is adjusted at a giveninstant, with current pacing parameters t_(c) and V_(c) corresponding atpoint S of coordinates {t_(c), V_(c)}.

Point L represents the minimum pacing energy value to be tested duringthe research phase, this point preferably being defined according to thepoint S (the position L is not fixed but depends on the currentstimulation energy):

{right arrow over (L)}=(α₁ t _(c), α₂ V _(c))

wherein α1 and α2 are constants lower than unity. Typical values for α1and α2 are, for example, α1=α2=2/3. Other values closer to zero couldhelp the search of points with lower energy, but with a longer searchphase (energetically more expensive).

In the case of loss of capture at the current point S (which is the casein the example of FIG. 5, since the point S is located below theLapicque's curve CL for the considered patient (the curve that definesthe border between capture zones ZC and non-capture zones ZNC)), arectangular window ADBC is defined, from both points A and B.

Point B is chosen such that:

{right arrow over (B)}=(β₁ t _(c), β₂ V _(c))

wherein β1 and β2 are constant superior to unity.

Point B establishes a maximum energy limit to be tested in the searchphase, which is energy dependent, as the minimum energy at the point L,on the position of the current point S. Point B is determined tocorrespond to an energy wherein it is certain that the stimulation willbe effective, which is the case if, for example, β1=4 and β2=2.

Point A is chosen as the middle of the segment LB:

$\overset{\rightarrow}{A} = \frac{\overset{\rightarrow}{L} + \overset{\rightarrow}{B}}{2}$

Point M is defined as the center of the rectangle ADBC:

$\overset{\rightarrow}{M} = \frac{\overset{\rightarrow}{A} + \overset{\rightarrow}{B}}{2}$

Point D of the rectangle ADBC is the point defined by t_(D)=t_(A), andV_(D)=V_(B), and point C is the point defined by t_(C)=t_(B) andV_(C)=V_(A) (ADBC the being a rectangle domain).

Four test points are defined to implement the search algorithm, namelypoints A, M, C and D. Point B will be considered a “rescue point” incase of detection of lack of capture. The device immediately applies acounter-stimulation with an energy corresponding to that of point B tocompensate for loss of capture and to be certain that thecounter-stimulation pulse is a capturing pulse.

The search for the best point of the four test points A, M, C and D isperformed in the order of increasing energy cost, with iterations of thesearch algorithm according to the following steps:

-   1) The standby point B′ of the possible next iteration of the search    algorithm is defined, which will be point B′=B;-   2) Point A is tested first because it costs less energy than the    other points D, M or C, the voltage and/or amplitude being lower in    A than in the three other points. Therefore stimulation with the    energy corresponding to the point A is applied;-   3) If a capture is detected during the test at point A, the    following points D, M and C are not tested, and a new rectangle    A′D′B′C′ is defined with B′=A, its center being M′;-   4) In case of lack of capture during the test at point A, we    calculate energy values proportional to the theoretical energy that    stimulation at points D, M and C cost, according to the formula    E_(p)(i)=V(i)*V(i)*t(i), i being a point among D, M and C.-   5) The three points D, M and C are classified according to the    values E_(p)(i) calculated in the preceding step, in descending    order, which gives three points X1, X2 and X3 such that:

[X1, X2, X3]=tri({D, M, C}), with E _(p)(X1)<E _(p)(X2)<E _(p)(X3)

-   6) Point X1 is then tested. If a capture is detected, no test is    carried out on the point X2 and X3 and a new rectangle is defined,    with B′=X1;-   7) In the opposite case, a counter-stimulation is applied (point B)    to compensate for loss of capture, and then point X2 is tested at    the next cycle;-   8) If a capture is detected at point X2, no test is performed on    point X3 and a new rectangle is defined, with B′=X2;-   9) Otherwise, a counter-stimulation is applied (point B) to    compensate the loss of capture and then point X3 is tested at the    next cycle;-   10) If a capture is detected at the point X3, a new rectangle is    defined, with B′=X3;-   11) If a capture was detected at one of the points X1, X2 or X3, the    above procedure of steps 1) to 10) is iterated, with B=B′ and    A=(L+B)/2, that is to say that A is the midpoint of segment LB′;-   12) If after any reiteration of test no point has produced capture,    then the search algorithm is terminated and the last point B that    produced the capture is defined as the optimal energy value.

In a simplified variant, the algorithm is stopped after the first testpoint which causes a loss of capture. The number of steps can thus bereduced, resulting in less energy consumed.

FIGS. 6 and 7 are representations of the algorithm of FIG. 5 applied toa first illustrative implementation (on FIG. 7, the isoenergetic curveswere added to the representation of FIG. 6).

Successive test points are numbered in the order 1, . . . , 9, and thepoints for which no capture was detected are shown by triangles in FIG.6.

It is noted that, in this example, after nine iterations the algorithmhas converged towards point 5 {0.8 V, 0.75 ms}, which will be the pointchosen as energy optimum. During these nine iterations, five points didnot cause a capture (points No. 4, 6, 7, 8, and 9), and the backup point(point 1) was used for the counter-stimulation.

In FIG. 8 another example is shown, wherein the algorithm convergesafter seven iterations, the point finally selected as the energy optimumbeing point 3 (the last point with capture).

In the simplified version of the algorithm mentioned above (whichconsists in stopping the algorithm from the first point that does notgenerate capture), the algorithm ends after only four iterations, thepoint being selected as the energy optimum being point 3, that is to sayin this case (but not necessarily, in general) the same point as in thefull variant of the algorithm.

What is claimed is:
 1. An active implantable medical device forstimulation, resynchronization and/or defibrillation, comprising: astimulation circuit adapted to deliver stimulation pulses to anelectrode in contact with a heart of a patient; a capture test circuitadapted to detect a presence or an absence of a contraction of the heartsubsequent to a stimulation pulse; and an energy adjustment circuitadapted to adjust an energy of the stimulation pulses delivered by thestimulation circuit by independently controlling a stimulation voltageand a pulse width of the stimulation pulse for each stimulation pulsedelivered; wherein the energy adjustment circuit is configured toimplement an iterative algorithm, wherein the iterative algorithm ateach current iteration comprises: setting a value of high energy,wherein the value of high energy includes a first pulse width and afirst stimulation voltage; setting a value of low energy, wherein thevalue of the low energy includes a second pulse width and a secondstimulation voltage, wherein the second pulse width is less than thefirst pulse width, and wherein the second stimulation voltage is lessthan the first stimulation voltage; delivering a stimulation pulse withthe value of low energy; and performing a capture test to detect thepresence or the absence of a contraction of the heart; and in thepresence of a contraction: ending the current iteration; andtransitioning to a new iteration, wherein the value of low energy is setas a new value of high energy; and in the absence of a contraction:applying consecutive counter-stimulation pulses of the first pulse widthand the first stimulation voltage set for the value of high energy; andselecting a last energy value that produced the presence of acontraction as a selected energy value.
 2. The device of claim 1,wherein implementing the iterative algorithm further comprises: defininga first intermediate energy value comprising the second pulse width andthe first stimulation voltage; defining a second intermediate energyvalue comprising the first pulse width and the second stimulationvoltage; defining a third intermediate energy value comprising a thirdpulse width between the first and second pulse widths and a thirdstimulation voltage between the first and second stimulation voltages;ranking the first, second, and third intermediate energy values bydecreasing energy value; and in the absence of a contraction afterdelivery of the stimulation pulse with the value of low energy:continuing the current iteration with delivery of pacing pulses insuccession with the first, second and third intermediate energy valuesordered by decreasing energy value until detection of a capture; and inthe presence of a contraction; ending the current iteration; andtransitioning to a new iteration, wherein the current intermediateenergy that produced the presence of a contraction is set as a new highenergy value; and in the absence of a contraction: selecting a lastvalue of energy that produced the presence of a contraction among thefirst, second and third intermediate energy values, as a selected energyvalue.
 3. The device of claim 2, wherein the third intermediate energyvalue is a value such that that the third pulse width is an average ofthe first and second pulse widths and the third stimulation voltage isan average of the first and second stimulation voltages.
 4. The deviceof claim 1, wherein the energy values of the stimulation pulsesdelivered by the stimulation circuit are at most equal to an upperenergy threshold value, wherein the first pulse width and the firststimulation voltage of the value of high energy of a first iteration ofthe algorithm is the upper energy threshold value.
 5. The device ofclaim 1, wherein the energy values of the stimulation pulses deliveredby the stimulation circuit are at least equal to a lower energythreshold limit value with a lower pulse width and a lower stimulationvoltage, wherein the low energy value is a value such that the secondpulse width is an average of the first pulse width and the lower pulsewidth and the second stimulation voltage is an average of the firststimulation voltage and the lower stimulation voltage.
 6. The device ofclaim 1, wherein the energy values of the stimulation pulses deliveredby the stimulation circuit are between an upper energy threshold valueand a lower energy threshold value calculated before each firstiteration of the algorithm.
 7. The device of claim 6, wherein an upperpulse width and an upper stimulation voltage of the upper energythreshold value and a lower pulse width and a lower stimulation voltageof the lower energy threshold value are calculated by applyingmultiplication factors, respectively above and below unity, to a currentpulse width and to a current stimulation voltage of the stimulationcircuit before a first iteration of the algorithm.
 8. A method fordetermining an energy for stimulation, resynchronization and/ordefibrillation, comprising: setting a value of high energy, wherein thevalue of high energy includes a first pulse width and a firststimulation voltage; setting a value of low energy, wherein the value ofthe low energy includes a second pulse width and a second stimulationvoltage and the second pulse width and second stimulation voltage areless than the first pulse width and first stimulation voltage;delivering a stimulation pulse with the value of low energy; andperforming a capture test to detect the presence or the absence of acontraction of the heart; and in the presence of a contraction: endingthe current iteration; and transitioning to a new iteration, wherein thevalue of low energy is set as a new value of high energy; and in theabsence of a contraction: applying consecutive counter-stimulationpulses of the first pulse width and the first stimulation voltage setfor the value of high energy; and selecting a last energy value thatproduced the presence of a contraction as a selected energy value. 9.The method of claim 8, further comprising: defining a first intermediateenergy value comprising the second pulse width and the first stimulationvoltage; defining a second intermediate energy value comprising thefirst pulse width and the second stimulation voltage; defining a thirdintermediate energy value comprising a third pulse width between thefirst and second pulse widths and a third stimulation voltage betweenthe first and second stimulation voltages; ranking the first, second,and third intermediate energy values by decreasing energy value; and inthe absence of a contraction after delivery of the stimulation pulsewith the value of low energy: continuing the current iteration withdelivery of pacing pulses in succession with the first, second and thirdintermediate energy values ordered by decreasing energy value untildetection of a capture; and in the presence of a contraction; ending thecurrent iteration; and transitioning to a new iteration, wherein thecurrent intermediate energy that produced the presence of a contractionis set as a new high energy value; and in the absence of a contraction:selecting a last value of energy that produced the presence of acontraction among the first, second and third intermediate energyvalues, as a selected energy value.
 10. The method of claim 9, whereinthe third intermediate energy value is a value such that that the thirdpulse width is an average of the first and second pulse widths and thethird stimulation voltage is an average of the first and secondstimulation voltages.
 11. The method of claim 8, wherein the energyvalues of the stimulation pulses delivered are at most equal to an upperenergy threshold value, wherein the first pulse width and the firststimulation voltage of the value of high energy of a first iteration ofthe algorithm is the upper energy threshold value.
 12. The method ofclaim 8, wherein the energy values of the stimulation pulses deliveredare at least equal to a lower energy threshold value with a lower pulsewidth and a lower stimulation voltage, wherein the low energy value is avalue such that the second pulse width is an average of the first pulsewidth and the lower pulse width and the second stimulation voltage is anaverage of the first stimulation voltage and the lower stimulationvoltage.
 13. The device of claim 8, wherein the energy values of thestimulation pulses delivered are between an upper energy threshold valueand a lower energy threshold value calculated before each firstiteration of the algorithm.
 14. The method of claim 13, wherein an upperpulse width and an upper stimulation voltage of the upper energythreshold value and a lower pulse width and a lower stimulation voltageof the lower energy threshold value are calculated by applyingmultiplication factors, respectively above and below unity, to a currentpulse width and to a current stimulation voltage of the stimulationcircuit before a first iteration of the algorithm.
 15. An activeimplantable medical device for stimulation, resynchronization and/ordefibrillation, comprising: circuitry configured to: deliver stimulationpulses to an electrode in contact with a heart of a patient; detect apresence or an absence of a contraction of the heart subsequent to astimulation pulse; and adjust an energy of the stimulation pulsesdelivered by the circuity by independently controlling a stimulationvoltage and a pulse width of the stimulation pulse for each stimulationpulse delivered; and determine a selected energy by applying a pluralityof iterations, wherein each current iteration comprises: setting a valueof high energy, wherein the value of high energy includes a first pulsewidth and a first stimulation voltage; setting a value of low energy,wherein the value of the low energy includes a second pulse width and asecond stimulation voltage and the second pulse width and secondstimulation voltage are less than the first pulse width and firststimulation voltage; delivering a stimulation pulse with the value oflow energy; and performing a capture test to detect the presence or theabsence of a contraction of the heart; and in the presence of acontraction: ending the current iteration; and transitioning to a newiteration, wherein the value of low energy is set as a new value of highenergy; and in the absence of a contraction: applying consecutivecounter-stimulation pulses of the first pulse width and the firststimulation voltage set for the value of high energy; and selecting alast energy value that produced the presence of a contraction as aselected energy value.
 16. The device of claim 15, wherein eachiteration further comprises: defining a first intermediate energy valuecomprising the second pulse width and the first stimulation voltage;defining a second intermediate energy value comprising the first pulsewidth and the second stimulation voltage; defining a third intermediateenergy value comprising a third pulse width between the first and secondpulse widths and a third stimulation voltage between the first andsecond stimulation voltages; ranking the first, second, and thirdintermediate energy values by decreasing energy value; and in theabsence of a contraction after delivery of the stimulation pulse withthe value of low energy: continuing the current iteration with deliveryof pacing pulses in succession with the first, second and thirdintermediate energy values ordered by decreasing energy value untildetection of a capture; and in the presence of a contraction; ending thecurrent iteration; and transitioning to a new iteration, wherein thecurrent intermediate energy that produced the presence of a contractionis set as a new high energy value; and in the absence of a contraction:selecting a last value of energy that produced the presence of acontraction among the first, second and third intermediate energyvalues, as a selected energy value.
 17. The device of claim 16, whereinthe third intermediate energy value is a value such that that the thirdpulse width is an average of the first and second pulse widths and thethird stimulation voltage is an average of the first and secondstimulation voltages.
 18. The device of claim 15, wherein the energyvalues of the stimulation pulses delivered by the circuity are at mostequal to an upper energy threshold value, wherein the first pulse widthand the first stimulation voltage of the value of high energy of a firstiteration of the plurality of iterations is the upper energy thresholdvalue.
 19. The device of claim 15, wherein the energy values of thestimulation pulses delivered by the circuity are at least equal to alower energy threshold value with a lower pulse width and a lowerstimulation voltage, wherein the low energy value is a value such thatthe second pulse width is an average of the first pulse width and thelower pulse width and the second stimulation voltage is an average ofthe first stimulation voltage and the lower stimulation voltage.
 20. Thedevice of claim 15, wherein an upper pulse width and an upperstimulation voltage of an upper energy threshold value and a lower pulsewidth and a lower stimulation voltage of a lower energy threshold valueare calculated by applying multiplication factors, respectively aboveand below unity, to a current pulse width and to a current stimulationvoltage of the circuity before a first iteration of the plurality ofiterations.